1. Field of the Invention
This invention relates to an improved method for the manufacture of shaped building blocks from selected coal combustion wastes and related products. More specifically, the invention relates to a method for producing high strength building blocks of precise dimensions and desired properties from coal combustion fly ash and bottom ash and related products.
2. Description of Related Art
Due to the rapidly diminishing availability of suitable landfill space and the escalating cost of land disposal, the daily generation of large volumes of coal combustion wastes at coal-fired power plants has created a costly disposal problem for electric utility companies throughout the world. Accordingly, power companies operating such plants have intensified their efforts to find high volume, income producing, commercial uses for coal combustion fly ash and bottom ash that provide economical alternatives to landfill disposal of these wastes.
There currently exists a number of known commercial uses for coal combustion fly ash and bottom ash as substitute materials in the production of low strength cementicious products. In one such use, the pozzoulanic properties of fly ash have established this waste product as a substitute for Portland cement in the production of concrete. The widespread use of fly ash in this application, however, requires that the carbon content of the ash be less than 3% by weight. The utilization of fly ash as a constituent in concrete is disclosed in U.S. Pat. Nos. 4,115,256, 4,121,945, 4,453,978, 4,517,078, 5,160,559, and 5,227,047, which disclose various means, including combustion, electrostatic, magnetic, air classification, and flotation means, for removing carbon from fly ash or chemical means to neutralize the negative effect of carbon on the properties of the concrete. As a result, the requirement for such reduced carbon content involves additional processing of the fly ash and undesirable associated capital investment and processing costs.
It is obvious from the foregoing disclosures that, in order to utilize large volumes of fly ash on a continuing basis, a commercial application must be found in which the carbon content of the fly ash has a less adverse effect upon the properties of the product in which it is incorporated. One such application, disclosed in U.S. Pat. No. 5,211,750, involves combining fly ash with flue gas desulfurization sludge, lime, and water, and compacting the resulting mixture into a shaped cementiciously-bonded product that is subsequently cured and crushed to yield an aggregate having a compressive strength in the range of 1,000-4,000 psi.
Another known method for utilizing fly ash as a constituent in a shaped cementiciously-bonded product, disclosed in U.S. Pat. No. 4,397,801, involves mixing (a) pulverized fly ash, (b) a spent fluidized bed combustion residue containing limestone particles and oxidic sulfur compounds, and (c) water, with or without the addition of Portland cement. The mixture is compacted to form a shaped masonry block which is subsequently crushed to create an aggregate used as a constituent in concrete, mortar mixes, and road base materials. The shaped masonry block produced prior to crushing is said to have exhibited compressive strengths that varied over time. The strengths were said to increase from 770-1,970 psi seven days after compacting to 1,570-2,950 psi twenty-eight days after compacting. Such strength levels are well below those required for high strength structural building bricks and blocks. U.S. Pat. No. 5,350,549 also discloses combining fly ash with spent fluidized bed material to form a shaped cementiciously-bonded composition. In this case, an aggregate is produced by curing the formed shape under ambient conditions while saturating the shape with water.
In yet another method for producing cementiciously-bonded construction products from coal combustion wastes, disclosed in U.S. Pat. No. 5,362,319, fly ash or bottom ash, with or without the addition of at least one other component selected from the group consisting of paper mill waste, lime, clay, Portland cement, or plaster of paris, is mixed with a strong oxidant to produce a shaped body that is allowed to harden and strengthen over time at ambient conditions.
A method of producing a stabilized cementiciously-bonded building block from both fly ash and bottom ash is discussed in U.S. Pat. No. 5,358,760. In this method, fly ash and bottom ash are combined with gypsum, lime, and calcium carbonate. The resulting admixture is formed into a block under a compressive force; and the block is allowed to cure under ambient conditions without the application of external heat. Construction blocks produced in this manner were said to exhibit a compressive strength of only 1,341 psi.
Another known method of utilizing both fly ash and bottom ash to produce shaped cementiciously-bonded structural products, disclosed in U.S. Pat. Nos. 5,374,307 and 5,366,548, involves adding water to a mixture of fly ash and bottom ash, which may or may not contain one or more additional waste products or sand or clay, and compressing the moistened mixture at a pressure in the range of 1,000 to 2,500 psi to form a molded body which is said to subsequently cure and strengthen at ambient conditions. Structures produced in this manner generally exhibited compressive strengths in the range of 1,900-3,600 psi.
These prior art disclosures produce shaped construction products that exhibit relatively low compressive strengths that increase slowly with time to ultimate levels that are inadequate for the use of these products as high-strength building brick or block. This lack of strength is believed to be due, primarily, to insufficient crystal formation and growth caused by curing the formed structures at ambient conditions, and the lack of control of temperature, pressure, humidity, and time during the curing process. The absence of significant crystal growth prevents the formation of a strong interlocking crystalline matrix. A typical cementiciously-bonded structure is that of a standard concrete building block. A structure of this type, which is typified by the lack of significant crystal formation and growth and the absence of a high strength interlocking crystalline matrix, normally yields a compressive strength of less than 3,500 psi. Such a structure, magnified 2,000 times, is shown in the photomicrograph of FIG. 1.
High strength, crystalline-bonded shaped building products produced from naturally-occurring materials, and methods for their manufacture, are known in the art. Calcium silicate bricks, produced essentially from sand and lime and intended for use in brick masonry applications, are disclosed in the American Society For Testing Materials specification ASTM C73-94a. Shaped building bricks and blocks of this type are produced on a commercial basis by Schneider Kalksandsteine GmbH in Germany and a number of other companies in Europe. Such bricks and blocks are known to be widely used in residential and commercial building construction in Germany, the Netherlands, and Russia.
Calcium silicate bricks are normally produced by mixing 85%-96% sand, 3%-8% lime, and up to 10% water, compacting the mixture to form a shaped body, and curing said body at an elevated temperature in pressurized steam for a time sufficient to allow the formation and growth of calcium silicate crystals. The compacting pressure employed normally ranges from 2,500 psi to 10,000 psi. Curing is normally accomplished by autoclaving the shaped bodies in an atmosphere of saturated steam at a temperature in the range of 180.degree. C. to 220.degree. C. and a pressure in the range of 200 psi-300 psi for a period of 4 to 6 hours.
Calcium silicate bricks produced from naturally-occurring materials under such conditions are reported to exhibit compressive strengths ranging up to 8,000 psi ("The Chemistry of Cements," Taylor, H. F. W., Academic Press). This dramatic improvement in strength has been attributed to the formation of a ubiquitous strong, interlocking crystalline matrix that tightly binds the mineral particles in the structure. It has been reported that the calcium silicate crystal phase most responsible for imparting strength to the interlocking crystalline matrix is Tobermorite, a hydrated calcium silicate species (Mortel, H., "Mineral Composition, Microstructure, and Physical Properties of Calcium Silicate Bricks," Fortschritte der Mineralogie, 58, pp 37-67, 1980). The structure (magnified 2,000 times) of a typical commercial calcium silicate building product manufactured from naturally-occurring materials showing the interlocking Tobermorite crystal matrix responsible for strengthening, is illustrated in the photomicrograph in FIG. 2.
U.S. Pat. No. 5,665,290 teaches that the addition of ground waste glass to a mixture comprised of a clay-containing material resulting from the burning of brown coal, quartz sand, lime, and water, support a pozzoulanic reaction that enhances the stability of the pressed brick shape produced therefrom during the drying prior to conventional firing of the dried brick.
The combination of compacting and steam-curing has also been used to produce high strength shaped building products from man-made wastes, such as copper mine tailings, zinc mine tailings, asbestos fines, and roofing granules (United States Department of the Interior, Bureau of Mines, Report of Investigation 7856, "Steam-Cured Bricks From Industrial Mineral Wastes," 1974). Building bricks of this type, produced by blending one of the aforementioned mineral wastes with calcium hydroxide or Portland cement, and compacting and steam-curing under conditions similar to those used to produce bricks from naturally-occurring minerals, are said to exhibit compressive strengths similar to those heretofore reported for bricks produced from naturally-occurring minerals.
Crystalline-bonded construction products incorporating coal combustion wastes and methods for their manufacture are also known in the art.
U.S. Pat. No. 4,683,006 discloses a low strength, crystalline-bonded shaped construction aggregate produced by mixing pulverized fly ash with lime, shaping the mixture, and curing the shaped mixture in a moisture-controlled atmosphere at temperatures in the range of 35.degree. C. and 80.degree. C. The strength of the resultant shaped body is attributed to a pozzoulanic bond that includes mechanical bonds formed by crystallites of Ettringite and amorphous calcium silicate, calcium aluminate, or calcium aluminum silicate, or mixtures thereof. The compressive strength of the shaped bodies so formed was said to be less than 100 psi.
U.S. Pat. No. 3,501,323 discloses a process for the manufacture of building, structural, and paving products based upon blending extremely fine siliceous material, a calciferous material, and water, molding the blend by compacting at pressures in the range of 10,000 psi to 20,000 psi, and hydrothermally treating the molded product at a temperature above 250.degree. C. for a total time at pressure of less than about 30 minutes. This patent teaches an average particle size of siliceous material in the range of about 4 microns to 48 microns (1,000-12,000 centimeters.sup.2 /gram) and a weight ratio of siliceous material:calciferous material of about 1:2 (33.3% by weight of calciferous material).
The use of fly ash as a partial substitute for sand in the production of calcium silicate-type brick has previously been reported in "Sand Fly-Ash Bricks," Materials Research Standards, USA (1964) and in Report 18 of the Research Association of Calc Sandstone (Germany), The Use of Industrial Waste Products, Their Influence On The Characteristics Of Calc Sandstone," Part II, Hard Coal Filter Ash, May 1970.
None of these prior art processes, or any product resulting therefrom, is adapted to utilize larger volumes of coal combustion wastes in the manufacture of shaped building products nor to significantly reduce the volume of coal combustion fly ash and bottom ash currently being disposed in landfills by permitting large volumes of such wastes to be utilized as the major raw material components in the manufacture of high strength shaped construction products. There remains, therefore, a need for a practical method for the manufacture of high strength, crystalline-bonded building bricks and blocks composed primarily of coal combustion fly ash and bottom ash. A process capable of producing such bricks and blocks would substantially reduce the need for landfill disposal of these wastes and be of great economic value to operators of coal combustion power plants.